As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the boards after soldering becomes more important. Generally, current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with a flux and thereafter passing this coated side of the board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes solder fusion. Commonly used fluxes consist, for the most part, of rosin, either used alone or with activating additives, such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the remaining flux and flux-residues are often removed from the circuit boards with a heated organic solvent. The requirements for such solvents are very stringent. Defluxing solvents should have the following characteristics: be low boiling, be nonflammable, have low toxicity and high solvency power so that flux and flux-residues can be removed without damaging the substrate being cleaned.
While boiling temperature, flammability and solvent power characteristics can often be adjusted by mixing different solvents together, the mixtures that are formed are often unsatisfactory because they fractionate to an undesirable degree on boiling while in use. The more volatile components of these mixtures vaporize first, causing the boiling mixtures to continuously change in both composition and boiling temperature. This fractionation is undesirable since it could result in altered solvency properties. The fractionation also makes it virtually impossible to recover and reuse a solvent mixture with the original composition.
On the other hand, an azeotropic mixture gives off vapor which has the same composition as the mixture. Since there is no preferential vaporization of the components, azeotropic mixtures remain constant boiling throughout distillation. In vapor-degreasing operations, this constant boiling aspect of azeotropic mixtures is desirable since the solvent vapor can be condensed back into a mixture with its original composition and then be used for the final rinse. Thus, vapor defluxing and degreasing systems act as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is a single material, an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distributions will result which could detrimentally affect the safety and efficacy of the cleaning operation.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes. This fact obviously complicates the search for new azeotropic compositions which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like compositions which have improved solvency characteristics and particularly greater versatility in solvency power.